1. Field of the Invention
The present invention relates to an image correction method and an image correction system for performing image correction.
2. Description of the Related Art
Color printers such as inkjet printers print color images in three color inks including cyan (C), magenta (M), and yellow (Y) inks or in four color inks including a black (K) ink in addition to the above three. Some printers are capable of changing the paper type (e.g., ordinary printing paper or surface-coated glossy paper (coated paper)), and the variety of paper types has increased along with the demands for improved image quality in recent years. When using an image forming apparatus of such a type, a user selects paper as appropriate from the above mentioned types to produce a desired image.
Incidentally, color images can be printed in their originally intended colors only when print heads discharged color inks in planned amounts, and if the amounts of color inks discharged vary, color reproducibility is lowered. Conceivable factors responsible for such variations in the amounts of color inks discharged include manufacturing tolerances of discharge units that discharge inks, and so on.
As a technique for correcting color reproducibility relating to these factors, a technique is disclosed that sets conversion characteristics for image forming conditions by performing printing on an image forming apparatus, obtaining color reproducibility as colorimetric data with a colorimeter, and comparing the colorimetric data with reference data (Japanese Patent No. 3867437). Also, another color correction technique is disclosed that performs gradation processing before color correction processing, because in the inkjet system, the number of dots deposited on paper does not show a linear change with the actual density (Japanese Patent Laid-Open No. 2000-301773). Also, Japanese Patent Laid-Open No. 2000-209450 discloses a method for creating a color correction table that is used to correct color conversion characteristics for certain image forming conditions into desired standard-color characteristics. Japanese Patent Laid-Open No. 2000-209450 discloses the technique for creating a color correction table by performing computations that can absorb a difference between different image forming conditions when the image forming conditions vary (Japanese Patent Laid-Open No. 2000-209450).
The general view of inkjet image forming apparatuses is that colors are determined by the printing paper type, the ink types, and the amounts of inks deposited per dot discharged from heads. Under such circumstances, the variety of the paper type has increased together with the recent improvement in image quality, and various paper types such as glossy paper and art paper are printable. However, because color-development characteristics vary from paper to paper, color-correction parameters that have been determined for a certain paper type are difficult to be used for other paper types for the purpose of achieving simple and high-precision color correction.
In Japanese Patent No. 3867437, since color reproducibility varies depending on image forming conditions, color reproducibility in an image forming apparatus is corrected under each image forming condition. With such a color correction technique, however, it is difficult to perform color correction under any image forming conditions other than those for which color reproducibility has already been corrected. As a result, a user of an image forming apparatus needs to perform printing and colorimetry under every available image forming conditions, which results in an increase in user workload and a lack of convenience. For example, the user's time and effort and cost increase with increased user workload and increased output paper waste.
Also, Japanese Patent Laid-Open No. 2000-301773 discloses a technique for performing color correction after every gradation processing performed by an image forming apparatus. With such a color correction technique, however, color correction needs to be performed for every execution of gradation processing as in Japanese Patent No. 3867437, which results in a lack of convenience as in the case of Japanese Patent No. 3867437.
In Japanese Patent Laid-Open No. 2000-209450, a dot-like printing material having multiple gradations for each element color is deposited on a printing medium and compared with standard colorimetric data so as to compute a color correction table. Moreover, if image forming conditions vary, parameters that can absorb a color difference between different image forming conditions are used to compute a color correction table. Such a technique, however, does not focus on the ink deposition amount discharged from a head. In inkjet printers, because the ink discharge (deposition) amount on paper does not show a linear change with a color change, it is necessary to acquire a colorimetric value for each gradation if gradations are different. In order to achieve high-precision color correction, it is thus necessary to print images with different ink deposition amounts on paper and perform colorimetry on those images. Unfortunately, this results in a lack of user convenience. In addition, it is also necessary to compute a color correction table, which increases the processing load required for computation.
With the above in view, a technique is required for performing color correction arising from individual differences among image forming apparatuses without sacrificing user convenience, in the case where image forming conditions such as the paper type vary in an image forming apparatus.